[flang] Allow forward reference to ENTRY from generic interface

The CreateEntry() function in name resolution needs to allow for the name
of an alternate entry point already having been declared in the outer scope
as the homonymous specific procedure of a generic interface; e.g.,

  interface foo
    module procedure foo
  end interface
  subroutine bar
    entry foo
  end subroutine

Differential Revision: https://reviews.llvm.org/D126436